Electrophoresis exploits the differential rate of migration of charged species through a separation medium, under the influence of an electric field, for purposes of separating and/or characterizing physical properties of the charged species. Typically, the sample containing the charged species to be separated is placed at one end of a separation channel (which may be a linear channel or a lane in a 2-dimensional slab) and a voltage difference is placed across opposite channel ends until a desired migration end point is reached. The separated analyte molecules may then be detected, e.g., by optical detection, radiography, or band elution.
As examples, gel electrophoresis in the presence of a charged surfactant, such as dodecyl sulfate, is widely used for protein separation and for characterizing protein molecular weight. Electrophoresis in a gel or liquid medium is commonly used to separate oligonucleotides with different numbers of bases, for example, in DNA sequencing.
One of the possible applications of microfabrication techniques that has been proposed is in the area of column separation devices, including electrophoresis devices. Jacobsen, et al. (Anal. Chem. 66:2369 (1994); Electrophoresis 16:481 (1995) have described a "microchip" electrophoresis device formed by etching an open electrophoresis channel, and suitable connecting reservoirs, on a glass slide. Because of the small chip dimensions, typically less than 10-15 cm on a side, it is necessary to form the separation column in the form of a serpentine pathway in order to achieve total column separation lengths suitable for most applications.
Although a serpentine column solves the problem of adequate column length on a microchip, it introduces a potentially serious limitation in terms of column resolution. When a electrophoretic band is migrating through a linear channel, the molecules making up the band, which are all migrating at roughly the same speed, tend to migrate as a tight band. However, the same molecules migrating through a turn in a serpentine pathway will migrate through the shorter inner side of the channel faster than through the longer outer side of the channel, leading to band spreading and nonuniformity across the width of the channel. At each turn in the pathway, more band resolution is lost. Heretofore, this problem has severely limited the range of practical electrophoresis applications in a microchip format.